A fuel cell stack generally has a configuration by stacking a plurality of membrane electrode assemblies via separators. The separators are provided to cause the respective stacked membrane electrode assemblies to be separated from one another as power generation units and to be electrically in direct connection with one another. Accordingly, each separator is placed between adjacent membrane electrode assemblies, such that the respective surfaces of the separator face an anode side and a cathode side of the adjacent membrane electrode assemblies. A reaction gas supply flow path for supply of a reaction gas (the air or hydrogen gas) is formed between the separator and the membrane electrode assembly. A seal line is formed around the periphery outside of the reaction gas supply flow path to prevent leakage of the reaction gas or a coolant. The seal line is generally formed by providing a gasket. The gasket has one end bonded to the separator and the other end pressed against the membrane electrode assembly, so as to prevent leakage of the reaction gas or the coolant. The process of manufacturing the fuel cell stack may perform not only a test for leakage of the reaction gas or the coolant from this seal line but a test for the adhesive state of the gasket forming the seal line. Patent Literature 1 proposes a non-destructive test for the gasket. A destructive test may, on the other hand, be performed to sample some of separators provided for producing the fuel cell stack and evaluate the sampled separators for whether the gasket is peeled off or what magnitude of force is required to peel off the gasket.